Heat a cast iron skillet, griddle, or double-sided sandwich maker. Spread mustard and mayonnaise on each piece of the bread, if using. Layer 2 slices each of the ham, turkey, salami, and cheese onto 2 pieces of the bread. Layer even amounts of pickle between the meat and cheese, and top each with even amounts of spinach, onion, and pepper. Top each with a piece of bread to make two sandwiches. Place sandwiches on the hot skillet or griddle. (If desired, you can use a small amount of clarified butter on the grill). Lay a heavy skillet on top of the sandwich to flatten the it to about one-quarter of its original size, grilling for about 2 to 3 minutes on each side. If using a double-sided sandwich maker, grill for 3 to 4 minutes. Serve sandwiches hot.